A capacitive touch panel often includes an insulator such as glass, coated with a conductive coating. As the human body is also an electrical conductor, touching the surface of the panel results in a distortion of the panel's electrostatic field, measurable as a change in capacitance for example. A transparent touch panel may be combined with a display such as a liquid crystal display (LCD) panel to form a touchscreen. A projected capacitive (PROCAP) touch panel, which may optionally include an LCD or other display, allows finger or other touches to be sensed through a protective layer(s) in front of the conductive coating.
FIGS. 1(a) to 1(g) illustrate an example of a related art projected capacitive touch panel, e.g., see U.S. Pat. No. 8,138,425 the disclosure of which is hereby incorporated herein by reference. Referring to FIG. 1(a), substrate 11, x-axis conductor 12 for rows, insulator 13, y-axis conductor 14 for columns, and conductive traces 15 are provided. Substrate 11 may be a transparent material such as glass. X-axis conductors 12 and y-axis conductors 14 are typically indium tin oxide (ITO) which is a transparent conductor. Insulator 13 may be an insulating material (for example, silicon nitride) which inhibits conductivity between x-axis conductors 12 and y-axis conductors 14. Traces 15 provide electrical conductivity between the plurality of conductors and a signal processor (not shown). ITO used for electrodes/traces in small PROCAP touch panels typically has a sheet resistance of at least about 100 ohms/square, which has been found to be too high for certain applications. Moreover, conventional ITO coatings for touch panels are typically highly crystalline and relatively thick and brittle, and thus in applications involving bending such ITO coatings are subject to failure.
Referring to FIG. 1(b), x-axis conductor 12 (e.g., ITO) is formed on substrate 11. The ITO is coated in a continuous layer on substrate 11 and then is subjected to a first photolithography process in order to pattern the ITO into x-axis conductors 12. FIG. 1(c) illustrates cross section A-A′ of FIG. 1(b), including x-axis conductor 12 formed on substrate 11. Referring to FIG. 1(d), insulator 13 is then formed on the substrate 11 over x-axis channel(s) of x-axis conductor 12. FIG. 1(e) illustrates cross section B-B′ of FIG. 1(d), including insulator 13 which is formed on substrate 11 and x-axis conductor 12. The insulator islands 13 shown in FIGS. 1(d)-(e) are formed by depositing a continuous layer of insulating material (e.g., silicon nitride) on the substrate 11 over the conductors 12, and then subjecting the insulating material to a second photolithography, etching, or other patterning process in order to pattern the insulating material into islands 13. Referring to FIG. 1(f), y-axis conductors 14 are then formed on the substrate over the insulator islands 13 and x-axis conductors 12. The ITO for y-axis conductors 14 is coated on substrate 11 over 12, 13, and then is subjected to a third photolithography or other patterning process in order to pattern the ITO into y-axis conductors 14. While much of y-axis conductor material 14 is formed directly on substrate 11, the y-axis channel is formed on insulator 13 to inhibit conductivity between x-axis conductors 12 and y-axis conductors 14. FIG. 1(g) illustrates cross section C-C′ of FIG. 1(f), including part of an ITO y-axis conductor 14, which is formed on the substrate 11 over insulative island 13 and over an example ITO x-axis conductor 12. It will be appreciated that the process of manufacturing the structure shown in FIGS. 1(a)-(g) requires three separate and distinct deposition steps and three photolithography type processes, which renders the process of manufacture burdensome, inefficient, and costly.
FIG. 1(h) illustrates another example of an intersection of ITO x-axis conductor 12 and ITO y-axis conductor 14 according to a related art projected capacitive touch panel. Referring to FIG. 1(h), an ITO layer is formed on the substrate 11 and can then be patterned into x-axis conductors 12 and y-axis conductors 14 in a first photolithography process. Then, an insulating layer is formed on the substrate and is patterned into insulator islands 13 in a second photolithography or etching process. Then, a conductive layer is formed on the substrate 11 over 12-14 and is patterned into conductive bridges 16 in a third photolithography process. Bridge 16 provides electrical conductivity for a y-axis conductor 14 over an x-axis conductor 12. Again, this process of manufacture requires at least three deposition steps and at least three different photolithography processes.
The projected capacitive touch panels illustrated in FIGS. 1(a) through 1(h) may be mutual capacitive devices or self-capacitive devices. In a mutual capacitive device, there is a capacitor at every intersection between an x-axis conductor 12 and a y-axis conductor 14 (or metal bridge 16). A voltage is applied to x-axis conductors 12 while the voltage of y-axis conductors 14 is measured (and/or vice versa). When a user brings a finger or conductive stylus close to the surface of the device, changes in the local electrostatic field reduce the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location. In a self-capacitive device, the x-axis conductors 12 and y-axis conductors 14 operate essentially independently. With self-capacitance, the capacitive load of a finger or the like is measured on each x-axis conductor 12 and y-axis conductor 14 by a current meter.
As described above, prior art transparent conductors 12 and 14 in touch panels are typically indium tin oxide (ITO), which is problematic for a number of reasons. First, ITO is costly. Second, thin layers of ITO have a high sheet resistance Rs (typically at least about 100 ohms/square); in other words the conductivity of ITO is not particularly good and its resistivity is high. In order for an ITO layer to have a sheet resistance less than 5 ohms/square, the ITO layer must be extremely thick (for example, greater than 400 nm). However, such a thick layer of ITO is both prohibitively expensive and less transparent. Thus, the high sheet resistance of thin layers of ITO limits their use in layouts requiring long narrow traces on touch panels, with an emphasis on large panels. Accordingly, it will be appreciated that there exists a need in the art for touch panel electrodes that are of material which does not suffer from the ITO disadvantage combination of high cost and low conductivity at small thicknesses.